Feature Request: support attribute query mapping

[MrPippin66]

It would be very useful if "systool" provided a query argument for attribute values.

The simplest example being finding all devices of a given class that have a current status

As an example, assuming using a --query option, you could do the following:

systool -c net --query operstate=up

This would simplify finding all devices that have a current status.

As an extension of this request, I'd suggest allowing compound queries which would include the following operations:

! = negation
&& = and
|| = or
() = nested conditional
<, <=, >, => = bound checking